Polarization mode dispersion (PMD) is one of the major obstacles in high-speed long-haul transmissions with bit-rates of about 10 to 40 Gb/s. In order to mitigate PMD, distributed polarization scrambling in conjunction with forward error correction (FEC) has been proposed in the paper “Experimental Demonstration of Broadband PMD Mitigation through Distributed Fast Polarization Scrambling and FEC” by X. Liu et al. in an ECOC 2004 post-deadline session.
The principle of this proposed method of PMD mitigation is shown in FIG. 2. An optical fiber line 1 for transmission of optical signals at a high bit-rate is arranged between an optical transmitter 2 and an optical receiver 3. At the transmitter end of the optical fiber line 1, a multiplexer 4 is arranged which performs wavelength division multiplexing (WDM) to optical signals with different wavelengths entering the multiplexer, forming a wavelength multiplexed signal which is transmitted through the optical fiber line 1 and demultiplexed in a demultiplexer 5 at the receiver end of the optical fiber line 1.
A plurality of fast polarization scramblers, of which three (6a to 6c) are represented in FIG. 2, are distributed successively along the fiber line 1. Each of the polarization scramblers 6a to 6c is followed by a successive fiber link 7a to 7c. The purpose of the polarization scramblers 6a to 6c is to provide scrambling signals which generate a periodic polarization change such that bit errors, which are generated by the interference of adjacent bit pulses of the optical signal transmitted through the fiber line 1, can be effectively reduced by error correction means in the receiver. (Commonly this means that the strong error periods are sufficiently short so that the FEC is capable to correct the errors.) The polarization scramblers 6a to 6c work at polarization modulation rates of some 10 MHz. A forward error correction decoder 8 is arranged in succession to the receiver 3 for decoding redundant bits being present in the optical signal in addition to information bits and compensating for transmission errors of the information bits by using the redundant bits.
In the presence of fiber PMD the distortion of the bit pulses (jitter, eye opening) changes quickly with the period of the scrambling rate. This means that for optimum PMD mitigation the receiver decision threshold (i.e. decision time and phase) has to follow the cyclic moving distortion. Hence the receiver 3 must be a so-called “scrambling receiver” which adapts to the eye changing with the scrambling rate.